1. Field of the Invention
The invention relates to a malfunction diagnostic apparatus and a malfunction diagnostic method for intake air temperature sensors.
2. Description of the Related Art
In an internal combustion engine provided in a vehicle, an intake air temperature sensor, which detects the temperature of intake air for the engine, is provided in an intake passage. A malfunction diagnostic apparatus determines whether a malfunction occurs in the intake air temperature sensor. Japanese Patent Application Publication No. 10-153125 and Japanese Patent Application Publication No. 2007-192045 describe examples of malfunction diagnostic apparatuses for the intake air temperature sensor.
In the malfunction diagnostic apparatus described in the publication No. 10-153125, it is determined whether a malfunction occurs in a downstream-side intake air temperature sensor that detects the temperature of intake air in the intake passage at a position downstream of a turbocharger, using a coolant temperature sensor that detects the temperature of a coolant of the internal combustion engine, and an upstream-side intake air temperature sensor that detects the temperature of intake air in the intake passage at a position upstream of the turbocharger.
More specifically, it is determined whether the upstream-side intake air temperature sensor normally functions, based on whether the intake air temperature detected by the upstream-side intake air temperature sensor is lower than the coolant temperature detected by the coolant temperature sensor by a value equal to or larger than a predetermined value, using the fact that the coolant temperature is generally higher than the intake air temperature at the position upstream of the turbocharger when the internal combustion engine is operating. It is determined that the upstream-side intake air temperature sensor normally functions based on the fact that the intake air temperature detected by the upstream-side intake air temperature sensor is lower than the coolant temperature detected by the coolant temperature sensor by a value equal to or larger than the predetermined value. Then, it is determined whether a malfunction occurs in the downstream-side intake air temperature sensor based on the intake air temperatures detected by the upstream-side intake air temperature sensor and the downstream-side intake air temperature sensor, on the condition that it is determined that the upstream-side intake air temperature sensor normally functions.
More specifically, it is determined whether a malfunction occurs in the downstream-side intake air temperature sensor, based on whether the intake air temperature detected by the upstream-side intake air temperature sensor is higher than the intake air temperature detected by the downstream-side intake air temperature sensor by a value equal to or larger than a predetermined value, using the fact that the temperature of the intake air at the position upstream of the turbocharger is lower than the temperature of the intake air at the position downstream of the turbocharger due to supercharging performed by the turbocharger. Thus, it is determined that a malfunction occurs in the downstream-side intake air temperature sensor, based on the fact that the intake air temperature detected by the upstream-side intake air temperature sensor is higher than the intake air temperature detected by the downstream-side intake air temperature sensor.
In the malfunction diagnostic apparatus described in the publication No. 2007-192045, it is determined whether a malfunction occurs in an intake air temperature sensor, using a coolant temperature sensor that detects the temperature of a coolant of an internal combustion engine when the engine is in a stopped state.
More specifically, when a time, which is required for a difference between the coolant temperature and the intake air temperature (i.e., the temperature of air in an intake passage) in the internal combustion engine to fall within a predetermined range, elapses after completion of the process of stopping the engine, it is determined whether the difference between the coolant temperature detected by the coolant temperature sensor and the intake air temperature detected by the intake air temperature sensor falls within the predetermined range. Thus, it is determined that a malfunction occurs in at least one of the coolant temperature sensor and the intake air temperature sensor, based on the fact that the difference between the detected coolant temperature and the detected intake air temperature is outside the predetermined range.
In the malfunction diagnostic apparatuses described in the publication No. 10-153125 and the publication No. 2007-192045, although it is determined whether a malfunction occurs in the intake air temperature sensor, it is not necessarily possible to make an accurate determination that a malfunction occurs in the intake air temperature sensor.
In the malfunction diagnostic apparatus described in the publication No. 10-153125, it is determined whether a malfunction occurs in the downstream-side intake air temperature sensor, using the coolant temperature sensor and the upstream-side intake air temperature sensor, when the internal combustion engine is operating and the amount of heat generated by the engine frequently changes. Thus, the coolant temperature and the intake air temperature frequently change due to the influence of a frequent change in the amount of heat generated by the engine. Therefore, it is difficult to accurately determine whether a malfunction occurs in the downstream-side intake air temperature sensor, based on the values detected by the above-described sensors (i.e., the detected coolant temperature, the detected temperature of the intake air at the position upstream of the turbocharger, and the detected temperature of the intake air at the position downstream of the turbocharger). Accordingly, even if it is determined that a malfunction occurs in the downstream-side intake air temperature sensor based on the values detected by the sensors in the malfunction diagnostic apparatus, there is a high possibility that the determination is not an appropriate determination.
In the malfunction diagnostic apparatus described in the publication No. 2007-192045, it is possible to determine that a malfunction occurs in at least one of the coolant temperature sensor and the intake air temperature sensor based on the values detected by the coolant temperature sensor and the intake air temperature sensor, when the time, which is required for the difference between the coolant temperature and the intake air temperature (i.e., the temperature of the air in the intake passage), in the engine to fall within the predetermined range, elapses after completion of the process of stopping the engine. However, when it is determined that a malfunction occurs, it is not possible to determine whether a malfunction occurs in the coolant temperature, or a malfunction occurs in the intake air temperature sensor. Also, if the above-described time required after completion of the process of stopping the engine (hereinafter, referred to as “soak time”) is decreased in order to increase the frequency of determining whether a malfunction occurs to a required level, the intake air temperature detected by the intake air temperature sensor may not sufficiently decrease due to, for example, solar radiation received at a portion of the vehicle, which is close to the intake air temperature sensor. In this case, even if the above-described soak time elapses, the detected intake air temperature has not sufficiently decreased, and thus, the detected intake air temperature is not appropriate for use in determination as to whether a malfunction occurs. Accordingly, in the malfunction diagnostic apparatus, it is difficult to make an accurate determination that a malfunction occurs in the intake air temperature sensor.